This invention relates to internal reflectance spectroscopy in which the IREs (internal reflectance elements) are cylindrical. The term "attenuated total reflection" (ATR) is often used in referring to internal reflection spectroscopy.
Although IREs having square cross-sections are widely used, circular cross-section IREs are becoming increasing popular, largely due to their (a) compatibility with reliable and convenient "O" ring seals, (b) advantageous fluid dynamic properties, and (c) compatibility with the circular cross-section IR beam of the typical FTIR spectrometer.
However, there has been considerable concern regarding the extent to which cylindrical IREs can be relied on for quantitative analysis. This concern was addressed by a detailed study carried out by Braue and Pannella on a commercially available IRE, in which mixtures of acetone and water were analyzed in five sets of three runs each carried out over a four month period. The experimenters' report is published in Volume 41, No. 6 (1987) of "Applied Spectroscopy" (pages 1057-1066). Although these authors concluded that precise quantitative analysis of aqueous solutions is possible with cylindrical IREs, they did observe some limitations, including (a) marked dependence of measurement results on positioning of the internal reflectance element, and (b) severe nonlinearity of peak height measurements at high absorbance values.
Although internal reflectance elements (both cylindrical and square cross-section types) are popular, several factors can lead to nonlinearity in their use for internal reflectance spectral measurements. Not least of these is the fact that the internal reflectance process itself becomes highly nonlinear at very high absorbance values, see Harrick "Internal Reflectance Spectroscopy" (1987), pages 22-23 Other sources of nonlinearity include chemical interactions and limited instrument resolution, and finally the dependence of measured absorbance on the angle of incidence of the IR radiation. This last factor is the primary subject of the present disclosure.
The internal reflection phenomena are inherently highly dependent on angle of incidence of the radiation at the interface between the internal reflectance element and the analyte. As can be seen from the computer-generated curves in Harrick's book (supra), a 10.degree. change in incidence angle can easily lead to a factor-of-two change in effective sample thickness and hence in measured absorbance. If radiation covering a range of angles is used, the results will be analogous to those obtained when a wedged cell is used in transmission spectroscopy. As has been discussed in detail by Hirschfeld in "Fourier Transform lnfrared Spectroscopy", Vol 2 (Ed Ferraro & Basile 1979), pages 193-239, this situation gives rise to a nonlinear dependence of measured absorbance on the concentration of the chemical being measured. In the case of the wedged cell, the data can, in principle, be corrected if the wedge thickness and angle are known. This is not the case in internal reflectance spectroscopy, since the dependence of effective thickness on the angle is nonlinear, and the angular distribution will generally not be well characterized. Thus, to obtain linear data using IREs, it is necessary to minimize the angular spread of rays traveling through the internal reflectance element.
The present invention is addressed to the deficiencies in the prior art devices. An early and structurally simple use of cylindrical IREs is shown in Wilks U.S. Pat No. 3,370,502, which discloses an IRE "cylinder or rod" having conical ends for entering and exiting radiation. In FIG. 3 of Wilks, "a cone 30 whose inner walls 31 are reflective is used to direct the incoming rays to the end 32 substantially perpendicular to the end surface thereof This arrangement is particularly suited for incoming parallel rays such as in spectrophotometers. The exit end 33 has a similar cone 34 for directing the rays to an indicating or recording portion of the spectrophotometer" (Col. 2, lines 50-56).
The Wilks design has major deficiencies, resulting from the wide range of angles at which different rays in the entering and exiting collimated beams strike the end surfaces and internal surfaces of the IRE.
Sting U.S. Pat. No. 4,595,833 relates to "reflaxicon optics" "for directing infrared radiation into the entry end of the cylindrically shaped internal reflection element, as well as for collecting radiation from the exit end of the element" (Col. 5, lines 11-14). Sting criticizes the device of the Wilks U.S. Pat. No. (3,370,502) in colume 4, lines 4-10: "This configuration is indicated to be particularly suited for incoming parallel rays (collimated source infrared radiation), such as in spectrophotometers. However, the funnel-shaped mirror optics undesirably have a wide variation in angle of incidence. Furthermore, difficulties arise in focusing the emergent infrared radiation onto the detector".
The rather complex device used by Sting in an effort to solve the problem of "wide variation in angle of incidence", which device is referred to in sales literature as the "Circle-Cell.RTM.", itself has a significant angle divergence problem. This is the device which Braue and Pannella (supra) used in their tests.
IRE (ATR) sampling systems may be designed for use either with collimated or with focused radiation. In the case of the Circle Cell.RTM., the focused beam in the spectrometer's sample compartment is intercepted and sharply focused onto the conical end of an IRE crystal. Within the crystal, rays can have incidence angles, at the interface with the analyte, ranging from typically 40.degree. to 49.degree.. With this wide range of angles, the effective sample thickness can vary as much as a factor of two. In addition, the rays which are more strongly absorbed at each reflection also experience the greatest number of reflections These two interrelated effects combine to give rise to a very strong dependence of absorption on incidence angle. Given the wide range of angles (40.degree. to 49.degree.) employed in the Circle Cell.RTM., these dual effects serve to seriously degrade linearity of the analytical data.